1. Field of the Invention
This invention relates to pneumatic isostatic forging of sintered compacts, including pneumatic isostatic forging of a rolling cutter adapted for use in a steel tooth rolling cutter drill bit utilized for drilling bore holes in the earth, the rolling cutter having a layer of wear resistant material, such as a hardmetal facing.
2. Description of the Related Art
It is well known to make sintered products by compacting metal powder, such as a plurality of iron, or steel alloy, particles in a die to form an unsintered compact, typically called a "green" compact, and then heating the green compact to a suitable temperature for a time sufficient to effect solid state bonding, or sintering, of the particles to each other. One type of compaction process is an isostatic process using a gas pressing medium known as the PIF process, which stands for pneumatic isostatic forging.
The PIF process has been used to densify sintered compacts wherein the sintered compact is encased in a shell which seals its outer surface against penetration of the gaseous pressing,medium into the interior of the sintered compact. The sintered compact is then heated back up to the sintering temperature and then may be surrounded by, and subjected to, pressing gas pressures sufficiently high (up to 60,000 PSI) as to densify the sintered compact. The sealing shell make take several forms, as are known in the prior art, including placing the compact in an evacuated thin flexible sheet metal can or mold; or applying a sealant, such as molten glass, electroless nickel, or an oxide sealant grown in situ on the surface of the compact to seal the surface pores.
It is also known to form densified sintered products which have a layer of wear resistant material to a portion of the external surface of the sintered product. An example of such a product is a rolling cutter adapted for use in a steel tooth rolling cutter earth boring bit utilized for drilling bore holes in the earth for the minerals mining industry. In the production of such rolling cutters, hardmetal inlays or overlays are employed as wearing and deformation resistant cutting edges and faying surfaces. These typically comprise composite structures of hard particles in a more ductile metal matrix. The hard particles may be metal carbides, such as either the cast WC/W2 eutectic or monocrystalline WC, or may themselves comprise a finer cemented carbide composite material. The hard particles which could be used include tungsten carbide, tungsten carbide/cobalt, titanium carbide, and commercially available ceramic carbides.
A major problem in forming densified sintered products, such as a rolling cutter having a layer of wear resistant material on it, utilizing a PIF, or pneumatic isostatic forging, process, is that the PIF process typically requires a relatively long period of time to sinter the green compact. This long sintering period, typically at an elevated temperature of approximately 2000.degree. F. will, it is believed, either destroy, or at least severely damage, any wear resistant material placed on the unsintered green compact, such as the hard particles previously discussed. In the case of a green compact formed of steel alloy powder and a tungsten carbide wear resistant layer, the prolonged heating, at elevated temperatures, leads to the steel alloy attempting to draw out, or suck out, the carbon atoms from the tungsten carbide wear resistant layer, thus either destroying, or severely damaging, the wear resistant layer. Additionally, some of the prior art techniques for sealing the outer surface of the sintered compact in the PIF process may have some disadvantage for some types of products. For example, additional manufacturing steps may be necessary to remove the sealing material, such as molten glass, nickel or oxide sealant, for the end product to be used. Additionally, such sealing materials could be a contaminant to some sintered compacts.
Accordingly, prior to the development of the present invention, it is believed that there has been no method of forming a densified sintered product from a metal powder, particularly when the sintered product includes a layer of wear resistant material thereon, by a pneumatic isostatic forging process, which: does not destroy, or severely damage, the wear resistant material; does not require additional manufacturing steps to remove a sealing material; and does not contaminate the sintered product with a sealing material. Therefore, the art has sought a method of forming a densified sintered product by a pneumatic isostatic forging process, which: does not destroy, or severely damage, a layer of wear resistant material that may form a part of the sintered product; does not require additional manufacturing steps to remove a sealing material; and does not contaminate the sintered product with a sealing material.